Conventional semiconductor lasers are formed of layers of III/V material, or alloys thereof, on substrate surfaces. Such layers form a cavity structure generally comprising successive layers of vapor phase epitaxy (VPE) or molecular beam epitaxy (MBE) deposited layers as follows: an n+type lower buffer layer an n-type lower cladding or confinement layer; an undoped active layer; a p-type upper cladding or confinement layer and a p.+-.-type contacting layer. Mirror facets are formed on both sides or ends of the layered structure, usually by cleaving the wafer perpendicular to the plane of the active layer. The two mirror surfaces and the active layer form a resonant optical cavity.
When current is applied across the layers, a non-equilibrium concentration of holes and electrons occurs in the active layer sandwiched between the upper p-type layer and lower n-type layer. When the holes and electrons recombine, photons are emitted out one of the faceted edges of the active layer in a direction perpendicular to the direction of current flow through the device. Such lasers can be categorized as laser edge-emitting devices.
Linear arrays of such edge-emitting devices have been formed in an attempt to achieve higher power levels than can be attained in a single device. Still higher power levels have been achieved by stacking and bonding these arrays to form composite two-dimensional arrays.
Those skilled in the art have also attempted to modify the geometry of light production in such devices so that the laser light is emitted in a direction perpendicular to the plane of the active layer or p/n junction to produce so-called surface-emitting lasers (SELs). Monolithic two-dimensional SEL arrays offer the opportunity for obtaining high power with high yield and reproducibility.
The SEL monolithic laser array devices fall into the following three general categories: